If you fancy the river route, as suggested by Jno, take a No 11 bus (from Victoria Coach Station) to Charing Cross Station. Walk down the left side of the station, to Embankment Pier.
From Marble Arch, take a 6, 13, 15, 23 or 139 to Charing Cross Station.
PS: I note that your question refers to 'Victoria bus station'. If you're going to be seeking directions, it might help to know that Victoria Bus Station is NOT the same place as Victoria Coach Station. (Victoria Bus Station is immediately in front of Victoria Rail Station. Victoria Coach Station is a 5 minute walk away).